In the semiconductor industry, there is a continuing trend toward producing semiconductor wafers having higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions in the wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. The basic lithography system consists of a light source, a photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask.
Conventional photomasks consist of chromium patterns on a glass or quartz plate which allow light to pass wherever the chromium is removed from the mask. In order to produce the photomask, a photoresist is initially formed above the chromium and patterned using known electron beam or laser beam techniques. Once the photoresist is patterned, the chromium layer is etched using a suitable etchant and the photoresist is then removed from the photomask.
Light of a specific wavelength is projected through the photomask onto a wafer coated with a resist, exposing the resist to the pattern formed on the photomask. In this manner the photomask can be used as a template for transferring the desired pattern onto multiple wafers. Exposing the resist on the wafer to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers which allows developer to dissolve and remove the resist in the exposed areas, presuming a positive resist is used. If a negative resist is used, the developer removes the resist in the unexposed areas.
Once the resist on the wafer has been developed, one or more etching steps take place which ultimately allow for transferring the desired pattern to the wafer. For example, in the event a hardmask is included below the resist on the wafer, a first etching step takes place in which the hardmask is etched to receive the pattern formed in the resist. In a subsequent etching step, the wafer is etched using the hardmask as a template for pattern transfer. The hardmask allows stronger etchants which are effective in etching the wafer to be used.
As the wafer feature size decreases, it becomes increasingly difficult to produce photomasks which can meet the accuracy demanded. If defects in the photomask are not accounted for, such defects are transferred to each wafer produced therewith, having a deleterious affect on the integrated circuits ultimately formed on such wafers. It becomes especially difficult to avoid defects from occurring in the photomask since such defects may cumulate from two different layers. For instance, any defect existing in the photoresist prior to etching of the chromium will be transferred to the chromium and also any additional defects which may exist in the chromium will exist as part of the photomask. Defects on these mask layers commonly arise from very small particles of foreign material, bubbles in the photoresist, or other flaws introduced during the pattern generation process. Additionally, such defects may result from subsequent handling following pattern formation.
To correct for such errors in the photomask, elaborate and expensive photomask inspections and processes for repair of mask layers have been developed. For instance, procedures for removing a defective mask layer and replacing it with a new mask layer have been used. Unfortunately, such procedures are time consuming and there is no way to guarantee that the newly formed mask will not again face similar problems.
It is also known to use ion beam sputtering or laser beam ablation to remove unwanted mask layer material from those areas of the photomask which were intended to be fully transmissive. Similarly, these techniques are also used for the deposition of additional, suitably opaque, material in those areas intended to be opaque. Unfortunately, these repair techniques typically yield either unacceptable results, or introduce undesirable phase or transmission defects in the final photomask. For instance, when attempting to repair an organic based resist material with an ion beam, typically there is produced a substantial amount of heat which deleteriously affects pattern repair abilities. An ion beam repair method is disclosed in U.S. Pat. No. 5,165,954 which is assigned to Microbeam, Inc.
Accordingly, there is a strong need in the art for a method of repairing lithographic photomasks which overcomes the drawbacks described above and others.